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Wastewater Treatment

Wastewater Treatment. Municipal WW Management Systems. Sources of Wastewater. Processing at the Source. Wastewater Collection. Reuse/Disposal. Treatment. Transmission and Pumping. Sources & Types of WW. Domestic Wastewater from residences, commercial & institutional

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Wastewater Treatment

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  1. Wastewater Treatment

  2. Municipal WW Management Systems Sources of Wastewater Processing at the Source Wastewater Collection Reuse/Disposal Treatment Transmission and Pumping

  3. Sources & Types of WW • Domestic Wastewater • from residences, commercial & institutional • flows ~70-90% of water supplied • Industrial Wastewater • highly dependent on industry • Infliltration/Inflow • enters through leaks, foundation drains, etc. • Stormwater • for combined sewers - largely in older cities

  4. Typical WW Characteristics

  5. 1972: Federal Water Pollution Control Act • PL 92-500 subsequently amended and now called the Clean Water Act • established water quality goals “fishable & swimmable” and timetable • established National Pollution Discharge Elimination System (NPDES) • construction grants for WW treatment • required secondary treatment (30/30) • 30 mg/L BOD5 • 30 mg/L TSS

  6. Primary Sedimentation Preliminary Treatment Biological Process Secondary Sedimentation Sludge Disinfection Sludge Conventional WW Treatment

  7. Preliminary Treatment • To remove materials that will interfere with subsequent treatment • Coarse Screening (bar racks) • Medium Screening • Comminution • Flow measuring • Pumping • Grit removal • Pre-aeration

  8. Bar Racks • Metal bars spaced a few cm apart across water flow • mechanical or manually cleaned • size of unit set by approach velocity • 0.6-1.0 m/s for mechanically cleaned • 0.3-0.7 m/s for manually cleaned • see Fig 10.4 and example 10.2 on pg. 311

  9. Grit Removal • Grit chambers intended to remove sand, cinders, gravel that may enter system by cracks in pipes, inflow etc. • Grit can cause excess wear in pipes and pumps • small sedimentation tanks; designed with the help of Stoke’s Law • no organics removal

  10. Grit Chamber

  11. Primary Sedimentation • Purpose: to remove suspended solids (smaller than grit, and less harmful) • Typical efficiency • 67% TSS removal • 33% BOD removal • Design parameters • overflow rate • weir loading rate • detention time Primary effluent is largely composed of soluble and colloidal organics which can be converted to settleable microbial solids and CO2 by biological treatment

  12. Primary Sedimentation • Primary Treatment • Removes ~50% of suspended solids

  13. Primary Sed. Tank

  14. Primary Clarifier: Center Feed

  15. Secondary Treatment • Generally includes some biological process plus secondary clarification • Required under PL92-500 • Converts soluble and colloidal organic materials to biomass and CO2

  16. Biological Treatment • Suspended Growth • Activated Sludge • Conventional, Extended Aeration, Contact Stabilization • Aerated lagoons • Aerobic digestion • Attached Growth • Trickling Filters • Rotating Biological Contactors

  17. Aeration Tank Air Secondary Sedimentation Return Activated Sludge Waste Activated Sludge Suspended Growth Systems Activated Sludge! Sludge

  18. Attached Growth: Trickling Filters Rotary Distributor Rocks or Plastic Media Air Underdrain With rocks, depth is limited to 2-3 m because of oxygen needs

  19. Rotating Biological Contactors • Drum diameters are typically 10-12 ft. • Rotation speed: ~1.5 rpm • May be in several stages • No flow recycle • Requires piloting

  20. Secondary Clarifier

  21. Sludge Disposal • Thickening • gravity, flotation • Digestion • aerobic, anaerobic • Mechanical Dewatering • Vacuum filtration, centrifugation, pressure filtr. • Disposal • land application, burial, incineration

  22. Anaerobic Digestion • Sludge held without aeration for 10-90 days • Process can be accelerated by heating to 35-40oC • These are called High Rate Digestors (10-20 days) • Advantages • low solids production • useable methane gas produced • Disadvantages • high capital costs • susceptibility to shocks and overloads

  23. Sludge Dewatering • Sludge drying beds • historically the most common • sand bed, 15-30 days, evaporation & seepage • Vacuum Filtration • cylindrical rotating drum covered with fabric • submerged with applied vacuum • Continuous belt filter presses (follows) • Plate pressure filters • vertical plates mounted on a frame

  24. Belt Filter Press (Komline-Sanderson)

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